A thin film transistor in which a semiconductor film is formed on an insulating surface is a semiconductor element that is essential to a flat panel display such as a liquid crystal display device or a light-emitting device and a semiconductor device such as an IC. Since there is limitation on manufacture of a thin film transistor in terms of allowable temperature limit of a substrate, a thin film transistor in which amorphous silicon that can be deposited at relatively low temperature, polysilicon that can be obtained by crystallization with use of laser light or a catalytic element, or the like is included in an active layer is mainly used for a semiconductor display device.
In recent years, as a novel semiconductor material which has a mobility as high as polysilicon and realizes such uniform element characteristics as are obtained with the use of amorphous silicon, a metal oxide having semiconductor characteristics which is called an oxide semiconductor has attracted attention. A metal oxide is used for various applications. For example, indium oxide is a well-known metal oxide and used as a transparent electrode material included in a liquid crystal display device or the like. Examples of such metal oxides having semiconductor characteristics include tungsten oxide, tin oxide, indium oxide, and zinc oxide. Thin film transistors including such metal oxide having semiconductor characteristics in its channel formation region have been proposed (Patent Documents 1 to 4 and Non-Patent Document 1).
As metal oxides, multi-component oxides as well as single-component oxides are known. For example, InGaO3(ZnO)m (m: natural number) having a homologous series is known as a multi-component oxide semiconductor including In, Ga, and Zn (Non-Patent Documents 2 to 4). Furthermore, it is confirmed that an oxide semiconductor including such an In—Ga—Zn-based oxide is applicable to a channel layer of a thin film transistor (Patent Document 5, Non-Patent Documents 5 and 6).